A typical process for making thin plates comprises continuously casting slabs, cooling the slabs, and then subjecting the once-cooled slabs to hot rolling. According to this process, however, it is necessary to heat the slabs which have been air-cooled after casting, and this process is disadvantageous with respect to energy consumption.
Recently, in view of the advantage that it can reduce energy costs markedly, a direct rolling process has been employed. The direct rolling process is a process for directly supplying cast slabs from a continuous casting machine to a hot rolling mill without intentional cooling. When thin cast slabs are used, it is possible to omit rough hot rolling steps from the direct rolling process. Currently, many attempts have been made to develop a new and practical continuous casting process for such thin slabs.
The direct rolling process using such thin slabs is advantageous because it is possible to omit rough hot rolling steps and because it is possible to achieve a less-energy consuming and more simplified process as a whole process for making steels in more efficient manner.
One such process for producing thin slabs is a process for casting slabs using a rectangular mold, in which a cast slab is rolled under a controlled roll pressure with controlled reductions using a plurality pairs of rolls while unsolidified portions remain in central portions of the slab. See Japan Unexamined Patent Application Laid-Open Specification No. 2-52159.
Using such processes in which cast slabs are reduced in thickness while unsolidified portions remain in central portions thereof (hereunder called "squeeze reduction"), it is possible to force out a molten phase enriched in solutes in the central portion of the slab upwardly to a mold so that cast slabs substantially free from segregation in liquid core can be obtained. Such segregation in the central portion is caused by the solidification of molten steel enriched in solutes. When the amount of rolling reduction during the molten state is adjusted, it is possible to manufacture thin slabs having a given range of thickness from slabs cast in a mold having a predetermined size of thickness.
According to a process disclosed in the above-mentioned JP-A 2-52159, however, cast slabs are produced with a rectangular mold, and tensile stresses are found along a solid-liquid interface in a longitudinal section during squeeze reduction. Such tensile strains sometimes cause cracking within the slabs.
Such a tendency is remarkable when the casting is carried out at a high speed and the reduction is relatively large. If there are many internal cracks in a cast slab, it is impossible to manufacture a rolled product through finish rolling. Thus, the reduction in thickness during squeeze reduction is restricted, and the merits of squeeze reduction cannot be enjoyed sufficiently when high speed casting is carried out.
On the other hand, as the casting speed increases, the pouring rate of molten steel into a mold through an immersion nozzle and the flow velocity are also increased, resulting in insufficient separation of inclusions within a residence time of molten steel in the mold. An increase in the amount of inclusions within slabs is inevitable. Thus, even if segregation in central portions can be forced out by squeeze reduction, it tends to be impossible to suppress an increase in inclusions, and clean steels free from internal defects cannot be obtained when the casting speed is high. In such a case, the advantages of squeeze reduction cannot be attained sufficiently.
In order to cope with an increase in casting speed, which is required recently, it is necessary to suppress the formation of internal cracks of cast slabs as well as segregation in the central portions, and it is also necessary to improve the cleanliness of cast slabs.